1. Field of the Invention
The present invention relates to an oxygen concentration detector which detects an oxygen concentration of a sample gas from, for example, burners for hot water or internal combustion engines.
2. Prior Art
An oxygen concentration detector comprising detector electrodes coated with aluminum oxide or the like is disclosed in JP-B-2-4858 (=U.S. Pat. No. 4,356,065).
However, the detector shows no region of constant current against voltage changes, i.e. flat current region, as shown in FIG. 13. Therefore, no good limit current characteristics cannot be obtained. FIG. 13 shows the data obtained with platinum electrodes coated only with alumina as a diffusion-resistant layer.
To overcome such a disadvantage, JP-A-61-45962 proposes to provide a zirconia diffusion-resistant layer on a zirconia element in view of differences in shrinkage and thermal expansion coefficient at the firing. In this case, a constant current region (flat region) is obtained as shown in FIG. 14, but occurrence of large hysteresis in the voltage-current characteristics is a problem as obvious from FIG. 14.